Recently in tires used in heavy load and high speed conditions, such as aircraft tires, radial structures have emphasized structural durability, running performance, fuel cost performance and the like. Conventional for aircraft radial tires use a structure wherein a cut breaker is provided between the carcass and a belt layer, where the carcass is tightened using a hoop effect. Since such aircraft tires are used under the conditions of high internal pressure, heavy load and high speed, a higher durability is required as compared with the tires used in other fields.
In particular, the bead part and the cut breaker end part are likely to be largely distorted due to deformation by the large load when taking off or landing. As a result, bead damages and breaker end damages frequently occur.
Based upon accumulated studies of the cause and effect of damage to aircraft tires, one of the primary causes was found to be in the cord arrangement of carcass and cut breaker plies. The so-called tire fabrics having mutually arranged cords (A) woven together with fine wefts (B), as shown in FIG. 14, are usually used as both plies. In the weaving positions of the wefts (B), as shown in FIG. 15, the cords (A) are disposed in a zigzag format throughout the thickness of the plies. As the thickness of each ply is increased, the cords (A) are extremely close to each other locally between plies. Accordingly, in the event of tire deformation, the shearing force acting between cords is concentrated in the areas where the cords are in close contact. Thus, and this portion becomes weak and induces cord looseness.
Along with an increase in the thickness of carcass plies, the internal temperature in the bead part, which is largely bent and deformed, rises further lowering the bead's durability.
To reduce breaker end damage, one method calls for placing a protective rubber between plies at the end of the cut breaker to absorb and relax the shearing force. However, the protective rubber locally increases the tread thickness on the shoulder, as compared with the tread thickness on the tire equator. As a result, the temperature in the shoulder area is raised, and the durability of the breaker end is lowered.
To prevent such nonuniformity of the tread thickness, rubber located along the outside of the belt layer is reduced to correspond to the thickness of the protective rubber. However, reducing the rubber increases the rigidity on the tread surface along the shoulder causing uneven wear.
One of the present applicants previously proposed, in Japanese Laid-open Document No. 1-314744, to use an elastic cord with a relatively large expandability as the carcass cord, having an elongation S.sub.5 (%) at 5 kg loading of 5 to 10, or the divided value D.sub.5 (%/d) of the elongation S.sub.5 (%) divided by the denier number of cords in a range of 7.35.times.10.sup.-4 to 14.7.times.10.sup.-4, so as to provide the carcass cord with a large elongation when inflating with an internal pressure. This is effective to reduce the compressive stress generated in the carcass cord of the bead part in the event of bend deformation, so that compressive breakdown of the carcass cords may be prevented. However, by using such carcass cord with large expandability, the shearing force acting between the cords increases due to the large elongation, and when using such cords, in particular, it essential to suppress cord looseness.